Interstellar dust

Suppose that the density of interstellar dust is uniform along the direction of the plane of the galaxy, but that its vertical density is given approximately by

[tex]\rho[/tex] = [tex]\rho[/tex] [tex]\o[/tex] exp - ( z / 0.15 )^2

where z is the distance from the plane of our galaxy in kpc. If the net extinction perpendicular to the galactic plane is 0.4 magnitudes (i.e. integrated from the mid-plane to infinity), find the extinction to an object in the galactic center, 8 kc away (and where z=0), and the ratio of the number of photons before and after the extinction